Infrared Spectroscopy on Equilibrated High-Density Amorphous Ice

High-density (HDA) and low-density amorphous ices (LDA) are believed to be counterparts of the high- and low-density liquid phases of water, respectively. In order to better understand how the vibrational modes change during the transition between the two solid states, we present infrared spectroscopy measurements, following the change of the decoupled OD-stretch (vOD) (∼2460 cm–1) and OH-combinational mode (vOH + v2, vOH + 2vR) (∼5000 cm–1). We observe a redshift from HDA to LDA, accompanied with a drastic decrease of the bandwidth. The hydrogen bonds are stronger in LDA, which is caused by a change in the coordination number and number of water molecules interstitial between the first and second hydration shell. The unusually broad uncoupled OD band also clearly distinguishes HDA from other crystalline high-pressure phases, while the shape and position of the in situ prepared LDA are comparable to those of vapor-deposited amorphous ice.


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of oxygen (and hydrogen) atoms in amorphous ice. In Figure S1B we compare the combinational mode of eHDA measured in the diffuse reflectance mode with ice V and VI measured at the same setup 2 . The FWHM of eHDA is broader than the FWHM of crystalline ices. The peak position of eHDA is shifted towards higher wavenumbers. Figure S1. Comparison of the uncoupled OD-stretch (A) and the combinational modes (B) of eHDA and high-pressure crystalline ices from Bertie et al 3,4 and Tonauer et al. 2 Data treatment: The OD-stretch mode overlaps with the strong bending and libration combination mode 3vR, v2+vR in the range of 2000 -2600 cm -1 . However, the main peak is well separated from the OD mode, which is sitting at the high-frequency wing. In order to analyze the OD spectra, we therefore subtracted a linear baseline in a range from 2262 cm -1 to 2600 cm -1 shown in Fig. S2A. In the resulting spectra ( Fig. S2B) we observe a decrease of the intensity due to some loss of the sample in vacuum. This is mainly observed during the heatingcooling cycle between the annealing temperatures and 80 K, where all spectra have been recorded. It can be seen as well that the intensity of CO2 becomes negative at higher temperatures, resulting from the sample loss. (More detailed, the software of FTIR spectrometer subtracts the background with initial larger amount of CO2 automatically after each measurement.) Since the HDA spectra obtained after heating to 90 K -115 K (blue curves) have similar broadness and peak positions despite getting lower in intensity, we relate

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at 80 K after heating to corresponding temperatures and annealing for 10min.) The last curves seem to have a linear increase towards higher wavenumbers. We relate it to light scattering from the copper grid, where the latter expands and shrinks at higher and lower temperatures, respectively, when we heat or quench the sample.

Comparison to vapor deposited ice (ASW):
In our previous work 5 we prepared vapor-deposited amorphous ice (ASW) which was found to be structural analog of low-density amorphous ice (LDA) derived from heating eHDA according to X-ray studies. We here compare the OD-stretch mode of eHDA, LDA derived from eHDA with ASW (Fig. S4). Unannealed ASW is known to be a porous material and due to that shifted towards higher wavenumber. Therefore, we compared our sample with both porous ASW (p-ASW) and LDA derived from annealing p-ASW to compact ASW (c-ASW).   An additional comparison to cubic ice (precisely to be called stacking disordered ice Isd, as not fully cubic) derived from heating vapor deposited ice to 160 K, is shown in Figure S5.
Interestingly the spectrum of the so derived crystalline ice (green solid line) is identical to the spectrum of crystalline ice after heating eHDA to 160 K (green dashed line). Both are clearly distinct from hexagonal ice prepared separately. All the spectra in Fig. S4 and S5 are measured at 80 K.
Comparison of combinational mode (vOH+v2, vOH+2vR) of ASW with LDA derived from eHDA is technically limited. Vapor deposited ASW samples are usually too thin for measuring relatively weaker mode at ~5000 cm -1 . We here compare a 5 µm thick ASW sample we had grown for 4 hours 5 . The comparison is presented in Fig. S6A, where we can see that the peaks positions of LDA and ASW are identical. Figure S6A: Comparison of combination mode of eHDA sample at 80 K and after annealing to 130 K, 160 K and ASW grown by vapor deposition 5 . All spectra were measured at 80 K.

Two different experimental FTIR techniques:
In the main manuscript we present Fourier-transform mid-infrared spectrometry (FTIR) spectroscopy measurements in transmission geometry on thin ice layers as well as measurements in diffuse reflection geometry using a Fourier-transform near-infrared spectrometer (FTNIR) 2 . Overlapping frequency range of the two instruments is the combinational mode (vOH+v2, vOH+2vR) at around 5000 cm -1 . The samples are prepared and measured independently at Stockholm University and University of Innsbruck, respectively. A comparison of eHDA is presented in Figure 2. Here we additionally compare eHDA as well as LDA ( Figure S6B). We observe a slight shift comparing the spectra, this is the diffuse Figure S6B: Comparison of eHDA and LDA measured by FTIR in transmission geometry (ice made as a thin layer) as well as diffusive reflectance on powdered bulk samples. All spectra were measured at 80 K. reflectance spectra has a larger FWHM than spectra obtained from transmission geometry measurements. The peak position for both, eHDA and LDA is shifted slightly towards higher wavenumbers than in transmission FTIR spectroscopy.

Coexistence of LDA and HDA:
In the main manuscript we report that coexistence of eHDA and LDA is observed at 120 K.
We here investigate further the fraction of the two components at different temperatures. We observe appearance of low-density structure at 90 K* (10 %), 105 K* (15 %), and 115 K* (30 %) in OD-stretch mode (vOD) shown in Fig. S7. (*Measurements are taken at 80 K after heating to corresponding temperatures and annealing for 10min.) Similar slow appearance of LDA fraction at lower temperatures were observed from X-ray diffraction measurements in Mariedahl et al. 6 using powdered eHDA samples. We could not apply the same analysis to the combinational mode (vOH+v2, vOH+2vR), since the transition affects the combinational spectra differently, due to different influence of bending and rotational vibration modes. For instance, it is known, that the FWHM of LDA of the combinational mode is larger than FWHM of eHDA, which is opposite to the OD-stretch spectra.